The Justice League is a team comprised of the premier superheroes of the DC Universe, making them "The Worlds Greatest Superheroes". The current incarnation of the League bears resemblance to the original Silver Age Justice League of America but features updated versions of its cast of characters for the DC Universe's current "New 52" continuity.

Have the parademons ever done something usefull?

Parademons are what makes up most of Darkseids army. They're big in numbers but individualy all they have is flight, heightened strength, and they carry apokoliptic weaponry. They're not very bright and only a few have earned the spotlight of a name and some intelligence but no more. Does anyone have any scans or anything where the parademons have achieved something.

Sure they can help with bank robberies or plot set ups (the parademons capture people and reprocess their flesh), but there is no way and no reason for them to succeed beyond that. It would make the hero look bad.

They are just cannon fodder villains. Like Ninjas or robots, demons etc.

The only difference is they are connected with Apokolips. Meaning they are really bad news, probably noting and apocalyptic like scenario will happen. They are a little stronger than your average minion, and travel in large numbers most of the time. They are grunts befitting an A-list super team.

They're generic moks that can be killed in the thousands with no one batting an eye. They're there to show Darkseid has control of a real army, one that can - with his help - take on even the mighty Justice League.